The U.S. Senate race in Kentucky could prove to be the most expensive Senate contest in history.

Credit Offices of Sen. McConnell and Sec. Grimes

A group that advocates for greater government transparency is challenging Kentucky’s U.S. Senate candidates to limit ads purchased by outside groups.

The Herald-Leader reports that Common Cause of Kentucky sent letters this week to the campaigns of Republican Senator Mitch McConnell and Democratic Secretary of State Alison Lundergan Grimes, asking them to sign a pledge aimed at limiting outside spending on the Senate race.

Under what the group calls the People’s Pledge, a candidate would agree to give to charity half of the cost of any ad bought by outside groups during the campaign. Grimes campaign manager Jonathan Hurst told the paper that Grimes has previously called for both campaigns to sign the pledge.

The McConnell campaign had not commented on the request by Common Cause of Kentucky as of Wednesday afternoon.

Kentucky’s Senate race will be one of the most closely-watched races in the country, with some analysts predicting it will also be the most expensive Senate race in history.